In large powered watercraft or vessels, the propeller shaft is often made of an exceedingly large size and strength in order to withstand the vibrations imparted to it from the propeller blades during operation. In addition, the propeller shaft is subjected to unbalanced moments stemming from the forces which are imposed by the sea on the portions of the propeller blades which are exposed or above the waterline during operation. These unbalanced forces can result in bending of the propeller shaft, and moreover, can produce resonant vibrations which can seriously damage not only the propeller shaft, but also its bearings.
With reference to sailing boats or yachts which utilize inboard auxiliary engines, a fixed propeller forms an unwarranted extra drag that impairs the sailing performance of the boat. In order to limit this degradation of performance, a number of various designs of special sailboat propellers have been advanced and some of these have been produced in quantity during the past years. Of these attempted technical solutions, two main groups are discernable.
One group consists of a so-called "feathering" propeller where the blade pitch angle can be changed, that is, an angular rotation of the blade along its radial axis out from the propeller shaft in a longitudinal direction of each propeller blade. In order to decrease the drag of the propeller when sailing, the blades are rotated (feathered) manually or automatically so that the blade surface is aligned with the oncoming stream to reduce the frictional drag when compared to that produced by typical "non-feathered" propeller blades.
The other group is constituted by so-called folding propellers where each of the blades of the propeller are folded rearward in the direction of flow when sailing. When the engine is started and the propeller shaft begins to turn, the centrifugal force makes the blades flap outward so that the propeller can work with the blades in a mainly radial position. The present invention in broad context only, may be considered as being related to this latter group.
The operational disadvantages with the earliest types of folding propellers have among other things been associated with the following problems: At low speeds the unfolding of the blades to operative position is not achieved due to the weight of the blades and the friction in the folding mechanism. Further, when the engine is started, it sometimes occurs that only one blade is extended causing a severe unbalance that often destroys the propeller shaft support bracket. Finally, the ability to provide thrust in reverse, especially the ability to brake at forward speed, is usually very poor in comparison with a fixed propeller. This deficiency is caused by the tendency of the blades to fold rearwards when the propeller thrust is reversed.
In attempting to solve or alleviate some of the above problems, the following arrangements have been used or proposed:
(a) Folding propellers with very thick blades in order to generate a high centrifugal force upon rotation to strive to counteract the tendency to fold during reverse speed.
(b) Providing the tips of the blades with cast-in magnetos in order to retain a folded blade position also at low speeds.
(c) Synchronizing the folding action of the two blades by means of intermeshing gear segments at the blade roots. This is to prevent asymmetric extension of the blades when the engine is started and also to prevent extension of one blade due to the weight of the blade when sailing at low speeds.
(d) Augmenting extension of the blades due to centrifugal force through a special mechanism that is actuated by the torsional moment of the propeller shaft.
None of the above attempts are believed to have adequately solved the problem, not only from the standpoint of operation but also, from the standpoint of providing an uncomplicated propeller structure that will withstand the forces of the sea while being durable and subject to manufacture at relatively low or practical cost.